=>ab-a-b=5
=>a(b-1)-b+1=6
=>(b-1)(a-1)=6
=>\(\left(a-1;b-1\right)\in\left\{\left(1;6\right);\left(6;1\right);\left(-1;-6\right);\left(-6;-1\right);\left(2;3\right);\left(3;2\right);\left(-3;-2\right);\left(-2;-3\right)\right\}\)
=>\(\left(a,b\right)\in\left\{\left(2;7\right);\left(7;2\right);\left(0;-5\right);\left(-5;0\right);\left(3;4\right);\left(4;3\right);\left(-1;-2\right);\left(-2;-1\right)\right\}\)
mà a,b là hai số nguyên tố lẻ
nên \(\left(a,b\right)\in\varnothing\)